1. Field of the Invention
This invention relates in general to a loudspeaker system comprising drivers of different frequency characteristics which are used in conjunction with the structure of the system for hemispherical radiation of sound for the different frequencies.
2. Description of the Prior Art
It is desirable to reduce amplitude-frequency-response distortion associated with the directional nature of typical piston type drivers forming loudspeaker assemblies. The frequency distortion associated with a piston type drive such as a cone loudspeaker, varies as a function of the radiating angle from the center axis passing through the center of the driver. Such distortion results from the relationship of the wavelength of the sound to be reproduced to the effective diameter of the piston creating the pressure waves of sound in air as that diameter is modified by the dispersion of sound through the diaphragm of the driver and the resonant modes of the diaphragm.
Conceptually, the amplitude of certain frequencies from a single loudspeaker will vary upwards and downwards as the angle from the axis of the driver is varied. The position of the low-amplitude values, called nulls, where sound of a particular frequency may be so low as to be inaudible, are often arranged nearly symmetrically around the axis of the loudspeaker, in an pattern undulating with the high-amplitude values, called lobes.
More specifically, as discussed in U.S. Pat. No 4,134,471 to Queen, the sound pressure radiated at a given frequency by a simple piston has a distribution pattern in space around the driver described by a type-1 Bessel function. This type-1 Bessel function distribution pattern has periodic lobes and nulls in the sound distribution pattern reminiscent of a (sin x)/x distribution of amplitude as a function of the angle from the axis of the driver. Because the lobes cannot coincide in space for every frequency, a listener at a single position will hear amplitudes ranging from lobes at some frequencies to nulls at others. From position to position, a listener will hear a different amplitude-frequency response.
Prior art loudspeaker systems attempted to reduce this null to lobe variation and achieve a large degree of wide angle sound directivity by the placing of many high- and low-frequency drivers on various surfaces of the enclosure of the loudspeaker system. If multiple drivers are employed, the overlap in their respective patterns is claimed to generate an overall pattern having fewer undesirable lobes. An example of such a system wherein multiple drivers are placed on the walls of the loudspeaker enclosure is shown in U.S. Pat. No. 3,590,942 issued to Globa. The shortcoming of such a configuration is the requirement for relatively large number of drivers within the enclosure and the probability that phase interference among the multiple drivers will create yet more nulls and lobes.
Another approach to creating wide-angle radiation of sound is to use sound reflectors in conjunction with a few direct-radiator type drivers to enhance the drivers' sound distribution capability, as shown, for example, in U.S. Pat. No. 3,819,005 to Westlund. Here, two drivers are mounted on a spherical shell juxtaposed to reflectors to increase the scattering of the sound from the drivers. This system is limited to the use of direct-radiator single-cone drivers or, in the alternative, coaxially-mounted low- and high-frequency drivers, combined in the space of each of the single-cone drivers. Typically, a single-cone direct-radiator driver has limited high-frequency response due to its size and mass. Because of this limitation, full-audio-spectrum-range radiation over a wide angle is not achieved. Alternatively, where a high-frequency driver is mounted coaxially, in the space of the single-cone low-frequency driver, to radiate into the reflector in conjunction with the low-frequency driver, the high-frequencies would scatter. However, this scatter is as a function of the increasing size of the structure, in a manner that creates cancellations of sound between the sphere and the reflector, thus reducing, particularly in larger structures, the ability of the nigh-frequency driver thus mounted to increase the bandwidth of the radiation.
A similar concept characterized by the use of a reflector in conjunction with multiple direct-radiator drivers is shown in U.S. Pat. No. 3,819,006 to Westlund. Here, a common reflector to three drivers housed in spherical enclosures is shown. Again, only a single-cone type of driver matched to the reflector is disclosed, thereby constraining the frequency response of the system to the limited frequency Performance ability of the single-cone driver arrangement.
Yet another mechanism for distributing sound while minimizing the presence of lobes as described above is the use of multiple horns. However, because of the physical dimensions of each of the multiple horns, the separate drivers have to be separated from each other, resulting in phase cancellations among the signals generated by the multiple horns. These phase cancellations at various frequencies will create inconsistent and non-uniform lobes acoustically similar to the ones discussed in conjunction with the (sin x)/x distribution from the axis of a driver. As described before, such lobes are undesirable as they severely distort the amplitude-frequency response of an audio signal at various points in space.
It is therefore an objective of the present invention to provide a loudspeaker system having two types of drivers, one of the low-frequency type and one of a high-frequency type, working concurrently with a single horn-like, sound-guiding structure having reflector characteristics for low frequencies and horn characteristics for high frequencies, thereby achieving generally hemispherical distribution of the sound waves radiated by both types of drivers.
Another objective of the present invention is to provide a loudspeaker system having drivers of multiple types working in conjunction with a single horn-like sound-guiding structure to achieve wide frequency response not limited by the frequency performance of a single driver.
It is yet another objective of the present invention to provide a loudspeaker system whose structure both supports the drivers mounted within, as well as forming an acoustical horn to direct sound waves uniformly over a wide angle.
It is yet another objective of the present invention to provide high-frequency sound efficiently and uniformly over a wide angle, typically a full hemisphere, while avoiding the presence of undesirable lobes in the sound pattern of the loudspeaker system.